A large water shield for dark matter, double beta decay and low background screening.

1. A large water shield for dark matter, double beta decay and low background screening. T. Shutt - Case R. Gaitskell - Brown

2. Water shields for dark matter or bb decay. • Conventional Pb + Poly shield for DM, bb decay expensive, inflexible at large size. • Ancient Pb (or Cu) to avoid 210Pb - $$. • Thick polyethylene - $$. • Higher intrinsic gamma background than water shield. • Existing water shields • SNO light water. • Borexino’s CTF: surrounds 2m Ø liquid scintillator • Boulby - UKDM • Liquid noble detectors: At a 1st order phase transition. • Hundred-kg LXe, LAr, bubble chamber modules not expensive. • Rapid evolution and scale-up to ton scale could happen very rapidly... …. if shielding weren’t prohibitive.

4. Multiple User Facility • Tom Bowles proposal at first Lead meeting, 2001. • Modular approach from 100 kg - ton scale for modular dark matter experiments. • Dual-phase detectors have some natural size limit (as opposed to XMASS/CLEAN/DEAP). • Modular approach will accommodate other experiments • Experiments may not have the same internal backgrounds. Spacing, arrangement. • Good platform for advanced screening • Ge counters • Beta cage, alpha screening. • Moderate-sized liquid scintillator.

5. Shielding summary Gammas: • 2 m ~ 105 expected from 20 cm Pb shield. • 4 m affords extraordinarily low background. • Final rate will depend on water purity. L. DeViveiros, R. Gaitskell, Brown

6. High energy neutrons from muons • Muons in rock, outside of veto • Low rate, but important • Cross section on hydrogen dropping (Mei and Hime, astroph/0512125) • Conversion in Pb multiplies them. N ~ 20.

7. High energy neutrons in water • Elastic scattering primarily on O. • But forward scattered • Overcome by simple thickness • 2m water better than feasible Pb/Poly shield • 4m water sufficient for 10-46 cm2 (~1 ton) sensitivity at 4850 mwe • Can we live at shallow depth? 10-46 cm2 WIMP rates 4850 mwe depth L. DeViveiros, R. Gaitskell, Brown

8. Water purity • Assumption: bulk contaminants will be very low with moderate cost commercial purification • 18 MΩ deionization • Radon is main question. • From initial water: let decay. (3.82 half-life). • From Ra. • Main concern of SNO • Borexino’s CTF: ~ 1 mBq/m3 with commerical system. • Make-up water. Membrane stripping/degassing. • Stable water • SNO, Kamland: should get stagnant water -> Rn decays. • Chiller with recirculation to enforce gradient. • Dark matter with discrimination may not drive high requirement. • Screening, other experiments may drive this.

9. From a proposal for Homestake DUSEL(R. Gaitskell, Brown / XENON) • 10 module system • 4 m shielding • Could be reduced to 3 • Cavern: 16m x 10m x 15 m. • Davis cavern +3m depth. 1.75 m 16 m 10 m

10. Mechanics • Detector grid hangs from ceiling, supports modules. • Detector modules either water-tight, or sealed in plastic • Feedthrough plate handles sealing of each module. • Muon veto: Based on CTF3, ~ 20 PMTs should give 99.9% or better efficiency. 14 m

11. Sealing against Rn • Cavern lined same as SNO cavern. 107 reduction. • Deck structure sealed to walls with flexible membrane. • Each detector module contains all conduit seals. • Use same mechanism for sealing against water. • N2 pure on blanket.

12. Where? • Possible “Early Implementation” at DUSEL. • Strong endorsement by both Homestake and Henderson DUSEL sites. • Implementation soon would provide very powerful boost to promising next-generation, very large scale detectors. • Tremendous opportunity for collaborative effort for liquid-noble gas DM detectors • SNOLAB?

13. Noble Liquid Dark Matter Consortium Adam Bernstein54David Cline54Rick Gaitskell54Yongsheng Gao54Andrew Hime10,18Ed Kearns10,18Dan McKinsey10,18,54Tom Shutt54Hanguo Wang54James White10,54Frank Wolfs54 So far: US based effort from CLEAN+DEAP, XENON, ZEPLIN. Open to further participation.

14. Consortium • Follows informal discussions over last ~1.5 years. • Previous DMSAG meeting catalyzed letter to committee. • First step: letter to DMSAG (6/26/06): • “We believe it would be beneficial to operate a US consortium, which could exploit common infrastructure and specific shared R&D projects. A prime example is a large multi-module water shield that could be used by a number of experiments (and also for ultra-sensitive low background screening).” • “In addition to benefiting the next phase of technical development, this consortium will also help lay the groundwork for what we anticipate to be a very large-scale experiment based on the noble liquid technology (or technologies) that prove most sensitive for detecting WIMP dark matter.”

15. PMTs • PMT radioactivity, g and n, is dominant issue in all experiments. • Idea: unified R&D effort with manufacturer(s) • Development efforts to date: • Hamamatsu: XMASS • ETL: ZEPLIN, DAMA, WARP • Larger effort may gain critical mass • Overlap of goals: • Radioactivity: common goal • Size: large, apart from top dual-phase array • Temperature: need extra metal coating for Ar/Ne.

16. Liquid-phase purification • Liquid phase purification needed at large mass • Heat load from gas phase: XENON10 rate -> 0.5 kW @ 100 kg • Common purifier technology: • “Spark-gap” or cold-getter • Ne can also use charcoal. • Key technical challenge: Clean fluid pumping

17. Internal neutron backgrounds • From PMTs, will become and issue below WIMP sensitivity of ~10-45 cm2 (nominal 100 kg active mass). • Common approach to mitigating makes sense, especially in context of water shield. • Outer liquid (or solid?) scintillator • Gd doping in water? • LAr/Ne shield? • Measurements to calibration of Monte Carlos?

18. Other possible joint R&D activities • Waveshifter. • Essential for Ar + Ne, may be good for Xe. • Cryogenics • DAQ • Rn screening • Monte Carlo • Nuclear recoil calibration techniques